1. Field of the Invention
The present invention relates to a transcoder for coded video and, more particularly, to a transcoder for coded video that enables conversion of the encoding mode at a high speed and with a high accuracy according to compressed coded data, itself, the compression of that is performed utilizing a transcoding technique, or, data that is obtained by decoding only a part thereof.
2. Description of the Related Art
As a conventional technique for changing the mode of encoding, there has been reported a method that transcodes from a digital video (DV) format to an MPEG format. When transcoding DV data having no motion vector information therein to MPEG data, two ways of transcoding are considered as being available, one of which is a method wherein transcoding to MPEG is performed in a state where there exists no motion vector information and the other of which is a method wherein transcoding to MPEG is performed by setting motion vector information.
In the former method (hereinafter referred to as “the first transcoding method”) wherein transcoding is performed without using any motion vector information, all data of the video are handled as intra-frame-coded frames (I-picture) and they are transcoded to MPEG data. In the latter method (hereinafter referred to as “the second transcoding method”) wherein transcoding to MPEG is performed by setting motion vector information, searching the motion vector is hierarchically performed by applying 1-dimensional inverse DCT transformation in both the vertical direction and the horizontal direction.
In the second transcoding method, first, an optimum position as viewed in the horizontal direction is determined with low-resolution 1-dimensional inverse DCT transformation and, with respect to that place, an optimum position as viewed in the vertical direction is extracted as the motion vector. In the searching in the vertical direction, an approximate image wherein interpolation has been performed using a technique of bilinear interpolation is used so that the motion vector can be extracted with an accuracy with units of a 1 pixel. Next, the same processing is repeatedly performed with high-resolution 1-dimensional inverse DCT transformation. And, finally, the motion vector is detected according to the image the DV data of that has been completely decoded. By using that motion vector, transcoding from DV to MPEG is realized.
The lower the bit rate is, the more prominent the image quality improving effect attainable with use of the motion vector is. Therefore, the first transcoding method that does not utilize the motion vector information has a problem in that it is impossible to transcode to low-bit-rate MPEG data. On the other hand, since the second transcoding method utilizes the motion vector, the point in problem that is inherent in the first transcoding method is solved. However, that method has another problem in that it takes a large amount of time to execute processing for detecting the motion vector. Namely, since partially performing inverse DCT transformation with respect to the DCT coefficients, reducing the processing load is limited because of utilizing inverse DCT transformation. In addition, since interpolation processing for generating an approximate image also needs to be executed, suppressing the amount of calculation is difficult.